1. Field of the Invention
This invention relates to creation of ultra-high vacuums utilizing cryopumps by the capture of gas molecules on extremely cold surfaces from enclosed volumes which have already been reduced to a very low pressure by mechanical or diffusion pumps. Cryopumping achieved its first major applicaton in the early stages of the space program where it was used in large space simulation chambers, rocket exhaust test chambers, and a low density wind tunnel. The advantages of cryopumping reside in providing a clean vacuum and achieving high pumping speeds economically in comparison to conventional vacuum pumping techniques, thus creating a continuing interest in improving cryopumping methods and apparatus.
2. Description of the Prior Art
Cryopumping (cryogenic pumping) devices have in the past used three surfaces to remove different gases from the vacuum environment. These surfaces generally have been cooled to various temperatures below 120.degree. K. These surfaces have been used to remove water and carbon dioxide (by freezing in the temperature range of 40.degree. to 120.degree. K.); nitrogen, oxygen, argon, carbon monoxide, methane and halogenated hydrocarbons (by freezing at temperatures between 10.degree. and 25.degree. K.) and helium, hydrogen and neon (by cryosorption at temperatures of 10.degree. to 25.degree. K.). Cryosorbtion is adsorbing gases in a sorbent at cryogenic temperatures. One type of three surface device is disclosed in U.S. Pat. No. 3,390,536. Patentee discloses a liquid cooled (nitrogen and helium) cryopump having three surfaces, the first of which is a removable surface radiation shield which blocks water vapor and carbon dioxide from contacting the second and third surfaces. All of the surfaces of the patentee's device are highly polished so that an appreciable amount of radiation is transmitted inside the device. This radiation can impinge on the third surface.
Another three surface device is disclosed in U.S. Pat. No. 3,579.998. Patentee discloses the traditional concept of using a chevron baffle to block water vapor and carbon dioxide from contacting the second and third surfaces. In addition, a second chevron baffle is used to prevent nitrogen, oxygen, argon, carbon monoxide, methane and halogenated hydrocarbons from contacting the third surface. The device is based on the geometry of the system and employs liquid cooling of the surfaces.
U.S. Pat. No. 3,168,819 discloses a diffusion pump cold trap which uses a single surface for cryopumping.
U.S. Pat. No. 3,338,063 discloses a variable area cryopanel but does not discuss a three surface technique for cryopumping.
U.S. Pat. No. 3,485,054 discloses operation of cryopumps in the range of from 10.sup.-1 to 10.sup.-5 Torr. This patent does disclose high thermal mass cryopanels to permit fast pump-down of a chamber from high initial pressure by enclosure of the cryopanels in a separate dewar that permits the cryopanels to remain cold while gases are being pumped out of the system.
U.S. Pat. No. 3,585,807 discloses a means for controlling the exposure of a cryopanel to heat and gas loads by a moveable shield panel.
Several technical papers have been presented which review cryopumping in the industrial environment. An article entitled "A Versatile Cryopump for Industrial Vacuum Systems" by J. Visser, B. Symersky, and A. J. M. Geraerts, which will appear in the publication Vacuum published by Pergamon Press Ltd., Great Britain, has been made available to workers in the field. This paper discloses using a closed cycle refrigerator for cooling three pumping surfaces in a cryopump.
An overall review of closed cycle refrigerators coupled to cryopanels can be obtained in the article entitled "Small Cryopump With Integral Refrigerator" by F. T. Turner and W. H. Hogan, appearing in Volume 3, No. 5 of the Journal of Vacuum Science and Technology, pages 252-257, published by the American Institute of Physics, New York, 1966.
Use of absorbent materials cooled to cryogenic temperatures is discussed in an article by P. J. Gareis and S. A. Stern entitled "Cryosorption Pumping of Helium and Hydrogen" pp. 26-30, Cryogenic Engineering News, October 1967, published by Thomas Publishing Co., Cleveland, Ohio.
In addition, technical literature is available from the large vacuum systems manufacturers such as Balzers, Veeco, and Varian, Inc.